Diesel engines and other internal combustion engines which require preheating frequently have glow plugs fitted into the cylinder. An arrangement to preheat such engines which, for convenience, will be referred to collectively as Diesel engines, is described in German Patent Publication DE-AS No. 1120 813. That system is directed to maintenance of the preheating temperature of the glow plug itself when air and fuel are supplied to the engine. Increasing use of Diesel engines for automotive vehicles required shorter and shorter times to permit starting of the engine, even under cold ambient temperatures. Glow plugs which use resistance wires with positive temperature coefficient characteristics are of particular interest for use with such engines--see, for example, German Patent Publication DE-AS No. 1 526 775, German Patent Disclosure Document DE-OS No. 1 426 173 and German Pat. No. 1 176 929. Systems and elements disclosed in these patent publications could not, satisfactorily, be installed in known circuit arrangements. The high turn-on current peaks resulted in damage to switching elements, particularly due to switch or contact chatter, so that switching contacts were endangered and subject to the possibility of welding together. Due to the high turn-on current peaks it was not possible to completely load the glow plugs to their maximum possible current carrying capacity.